Both trans-1,2-dichloroethene [CAS 156-60-5] and cis-1,2-dichloroethene [CAS 156-59-2] are valuable compounds having a variety of uses. They may be used separately or together in various proportions as low temperature extraction solvents for heat sensitive substances, as solvent for the manufacture of rubber solutions, and as coolant in refrigeration plants. Each compound has been copolymerized with other ethylenically unsaturated monomers to form copolymers.
1,2-Dichlororethene [CAS 540-59-0] can be produced by direct chlorination of acetylene at about 40.degree. C. to about 80.degree. C., by the reduction of 1,1,2,2-tetrachloroethane, by the pyrolytic dehydrochlorination of 1,1,2-trichloroethane, or as a by-product in the chlorination of chlorinated compounds. However produced, the 1,2-dichloroethene comprises a mixture of the trans and cis isomers, the proportions of which depend upon the conditions of manufacture.
For some uses, trans-1,2-dichloroethene or a trans-rich mixture of the two stereoisomers is preferred. Trans-1,2-dichloroethene is more reactive chemically than the cis-isomer, especially in 1,2-addition reactions. It also has a lower normal boiling point and a lower latent heat of vaporization than the cis-isomer, thereby favoring the trans-isomer or a trans-rich mixture for extractions where it is desired to recover the 1,2-dichloroethene by distillation for recycle. The trans-isomer also has a lower viscosity than the cis-isomer, so that less energy is required for pumping the trans-isomer or trans-rich mixture of the two stereoisomers.
For other uses, cis-1,2-dichloroethene or a cis-rich mixture of the two stereoisomers is preferred. Since the cis-isomer has a lower melting point than the trans-isomer, the cis-isomer or a cis-rich mixture is better suited for use as an indirect heat transfer medium in refrigeration systems operating at low temperatures. The solubility of the cis-isomer in water at 25.degree. C. is less than that of the trans-isomer, so that the cis-isomer or a cis-rich mixture of the stereoisomers is preferred for some extractions where an aqueous phase is present. The lower chemical activity of the cis-isomer is advantageous where chemical stability is desired.
It is therefore desirable in many instances to isomerize cis-1,2-dichloroethene to trans-1,2-dichloroethen or vice versa, depending upon the intended use of the product.
Processes are known for effectuating the isomerization. In the presence of bromine or alumina at high temperatures, one isomer can be partially converted to the other. The direction of the reaction depends upon the relative concentrations of the isomer in the reaction mixture as compared with the equilibrium concentrations at the conditions of pressure and temperature prevailing during the reaction. At 825.degree. C., for example, the equilibrium mixture contains about 55 percent of the cis-isomer, while at 975.degree. C. the proportion of the cis-isomer falls to 52 percent.